Get a man in with a really big stump grider and chipper. Saves a lot of effort. Alternatively, do what we did and have the Leylandi cut off at 6 ft, kill the remainder with professional tree stump killer and rotavate the ground between the stumps to allow for the planting of Laurels. The Laurels were about £22 each for bushy 4 ft tall plants at todays prices and were in 20L? containers. Plants Galore supplied them. They have established well, with no failures. Cost about £2K in total IIRC. I had to lay a drip watering hose across the Laurels in the first year as the weather was dry. The Leylandi were used as the mounting posts for a chain link fence, this was the reason for retaining the stumps. I reckon they will rot out in less than 20 years and be replaced with Metpost mounted wooden units. Our Leylandi were on a .7M Bank, behind a brick retained raised bed. Sounds similar. The method outlined did not disturb the soil at all as the tree roots are not touched much at all. Laurels have all the characteristics of slow growing weeds and are now up to about 5,5 ft providing a perfect screen!